The call by the Sunni-based Iraqi Islamic Party on Wednesday comes after revelations that about 170 detainees, mostly Sunni, were illegally held at a centre run by the Shia-dominated Interior Ministry, in a case likely to embarrass the US military supervising local security forces.
"We insist on having an international investigation," Islamic Party spokesman Alaa Makki said.
"There have been similar cases in the past, and investigations into them led to nothing," said another party spokesman, Ayad Samarrai.
"We want an international and impartial inquiry as we are beginning to think there are people high up in government who are responsible, or at least accomplices."
US forces blamed
Makki also blamed US-led forces for the abuse, saying it could not happen without "their green light".
Prime Minister Ibrahim al-Jaafari has ordered an investigation into the allegations, with a committee due to report its findings within two weeks.
Al-Jaafari has ordered an
investigation into the allegations
Al-Jaafari's spokesman Laith Kubba told the BBC the prime minister was "outraged" by the alleged abuse, adding: "It goes pretty much against all the core values that this government holds."
The case came to light after US forces raided the underground facilities of an Interior Ministry complex in south Baghdad on Sunday.
Hussein Kamal, Iraq's deputy interior minister, told CNN television he saw evidence of torture.
"I saw signs of physical abuse by brutal beating. One or two detainees were paralysed. And some had their skin peeled off various parts of their body."
The Association of Muslim Scholars, the main Sunni religious organisation in Iraq, accused Interior Ministry services of "resorting to torture and ransoming prisoners".
Committee spokesman Shaikh Abd al-Salam al-Kubaissi said that his organisation had "filmed testimony of released detainees who had been tortured" and that the videos were handed over to Arab League chief Amr Mussa when he visited Baghdad last month.
Videos of torture testimony were
given to the Arab League chief
Al-Kubaissi also accused Interior Ministry services of "detaining people at night in their homes on terrorist charges and then torturing them into making confessions, parts of which are then broadcast on television".
Some detainees were released a month or two later "after paying a bribe, with no charges being brought against them", al-Kubaissi added.
The US embassy and US military issued a statement welcoming the Iraqi government's decision to investigate the case and stressing that the mistreatment of detainees was "totally unacceptable".
US Ambassador Zalmay Khalilzad and General George Casey "have discussed this case with the leaders of the Iraqi government at the highest levels", the US embassy statement added.
Abu Ghraib echoes
A scandal over the abuse by US forces of Iraqi detainees at Baghdad's Abu Ghraib prison last year led to international condemnation of the US as it struggled to defend the March 2003 invasion and occupation of Iraq.
The US will provide "technical assistance, including support from US law enforcement elements of the department of justice and the Federal Bureau of Investigation" to help the Iraqi government look into the matter, the US statement added.
The revelations come a month ahead of general elections for a permanent government, the final stage in Iraq's transition to democracy after the toppling of Saddam Hussein.
The US army was criticised for
Abu Ghraib prisoners' abuse
In London, British Foreign Secretary Jack Straw spoke of his "deep shock" and said: "Whoever carried out this abuse must be caught and brought to justice, regardless of rank or background."
The UN mission in Iraq on Monday accused the Interior Ministry of holding hundreds of people in detention despite judicial orders for their release.
Sunni Arabs, who provide the backbone to the anti-US fight, have repeatedly accused Iraq's security agencies, some of which have allegedly been infiltrated by Shia militias, of engaging in torture and extra-judicial executions.
The European Union has added its concern over the alleged abuse at the detention centre, saying it showed the need to bolster human rights in the violence-scarred country.
EU external relations commissioner Benita Ferrero-Waldner noted that al-Jaafari had ordered an investigation into the reports and stressed that the facts were not confirmed.
But she said: "If it were so, it's clear ... that human rights are part of our very, very strong values, and those values that we want also to export to Iraq."
Speaking at the European Parliament in Strasbourg, she recalled that the EU agreed this year to fund training of Iraqi judges and senior police.
"What we can do is to contribute ... by building up a police corps that does not torture, a police corps that really knows where the limits are, but at the same time tries to restore order," she said.